This invention relates to an adhesive coating which has a dry, tack-free surface convertible to a permanently pressure-sensitive adhesive layer by the application of heat. The invention is useful for labeling, sealing, adhesive layer transfer, decalcomania, and other packaging applications.
Non-tacky coated products which can be converted by heat to a tacky adhesive product have been described in prior art. Perry in U.S. Pat. No. 2,462,029 describes adhesive compositions containing discrete solid particles of a plasticizer dispersed in a potentially adhesive film. Heating the film activates the adhesive by causing the plasticizer to melt and merge with the polymeric components of the film. This results in an adhesive product which will retain its tack over an extended period of time. Typically this merging of the plasticizer and polymeric material results in increased tack or viscidity, and also lowers the temperature at which the composition solidifies to a dry, non-tacky film and conversely, at which temperature such dry film, after solidification, softens or activates upon a subsequent heating. The compositions of Perry have proved to be extremely useful in providing adhesive films and articles having such films, for example tapes and labels, which are dry and non-tacky at normal room and storage temperature conditions, but are heat activatable to adhesive tackiness and retain such adhesive tackiness when subsequently cooled to temperatures below their initial activation temperature. After initial activation, these films can be allowed to cool to temperatures appreciably below the activation temperature and can subsequently be applied to the desired surface. Heat need not, therefore, be maintained after the initial activation to keep the adhesive ready for use. Compositions of the Perry invention are limited to the use of solid crystalline plasticizers which due to their solid nature prior to initial activation remain inert within the composition.
Lawton et al. U.S. Pat. No. 3,104,979 describes an improvement over the Perry composition wherein liquid plasticizers are used in the place of the solid plasticizers of Perry. These liquid plasticizers are incorporated as solid inclusion compounds with urea. Heat forces the liquid plasticizer from the channels in the solid urea crystal and this liqiud plasticizer merges with the thermoplastic polymeric components of the film. The potentially adhesive film is dry and non-tacky when the plasticizer is in the included state and becomes tacky when the plasticizer is released from urea.
Both systems have a disadvantage in that both systems will detackify on cooling or ageing. The Perry plasticizers recrystallize and the Lawton plasticizers will recomplex with urea. There have been reports of numerous examples of the Perry type labels falling off the substrates during cold winter conditions. The adhesion of these systems is restricted to a limited number of substrates primarily due to lack of flexibility in adhesive formulations.